The present invention is generally related to the collection of gas from subterranean formations, and more particularly is directed to a method for collecting gas from subterranean formations having a plurality of spaced apart seams containing the gas.
Many subterranean formations may contain gas. As an example, coal seams, or deposits, generally include a significant amount of methane gas which escapes therefrom as the coal is mined, thereby causing hazardous conditions in underground mining operations. Previous attempts to remove methane from underground coal seams has been primarily directed to removal of such gas in order to provide a safe working environment for the mining of coal.
A number of methods are employed to reduce the methane level in working mines. These methods include air dilution systems to provide sufficient air within the mines to reduce the methane level below 1% to prevent a combustive mixture from forming, drilling of vertical shafts from the earth's surface to intersect the seams in advance of mining, and the drilling of holes within the coal seams in advance of mining either from the earth's surface or from an adjacent coal seam. As an example of these methods see U.S. Pat. No. 3,934,649 to Pasini et. al., entitled "Method For Removal of Methane From Coalbeds" and U.S. Pat. No. 4,303,274 to Thakur entitled "Degasification of Coal Seams".
Heretofore, there has been no system or method for the recovery of methane gas from underground, or subterranean, formations irrespective of later mining of the seams for their coal content. The present invention is directed toward a gas drainage system, such as for methane, for collecting such gas from low pressure reservoirs, such as virgin coal seams, through the use of horizontal boreholes completed from a multiplicity of levels. The method is also suitable for removal of methane gas from coal seams having thicknesses less than that required for commercial mining of coal from the coal seam.